1. Field of the Invention
The present invention relates to an improvement in the semi-submerged ship. More particularly, the invention relates to an improved semi-submerged ship in which oscillations of the ship are remarkably eliminated while the ship is stopped.
2. Description of the Prior Art
As the semi-submerged ship recently attracting attentions in the art, for example, the semi-submerged catamaran, there is known a semi-submerged ship comprising an upper hull which is located above the surface of water and is used for passengers or cargos or as a floating factory and two lower hulls which are located below the surface of water and cover the majority of the displacement, wherein the upper hull is connected to the lower hulls through two struts arranged vertically across the water surface, which cover the minority of the displacement.
The structure of a typical instance of this conventional semi-submerged ship is illustrated in FIGS. 1 and 2. In this structure, two lower hulls 1 always located below the surface of water are arranged substantially in parallel to each other with respect to the direction of advance, and an upper hull 3 always located above the surface of water is connected to the lower hulls 1 through at least one relatively thin streamlined strut 2 mounted vertically on each of the lower hulls 1. In this semi-submerged ship, either the full load draft or the light load draft (unload draft) line L is on the struts.
Since the water-line area of this semi-submerged ship is smaller than that of an ordinary ship, the wave-making resistance is low and a required power at a high speed navigation is reduced. Namely, the semi-submerged ship is excellent in the high speed performance. Moreover, the semi-submerged ship is excellent in the motion characteristics on waves and the deck area can be broadened. Therefore, the loading efficiency and operation efficiency can be improved.
For example, the oscillation, one of the motion characteristics in waves will now be discussed.
The natural period Tm of the heaving motion of the ship is theoretically determined by the following equation: ##EQU1## wherein Tm stands for the natural period of the heaving motion of the ship, V stands for the displacement volume (m.sup.3), Aw stands for the waterplane area (m.sup.2) (the sectional area of the ship on the plane of the draft line), .pi. stands for the ratio of the circumference of a circle to its diameter and g stands for the acceleration of gravity.
As is seen from the foregoing equation, the natural period of the heaving motion of the ship is determined based on the waterplane area of the ship, and if the displacement volume is the same, the smaller is the waterplane area, the longer is the natural period of the ship motion and the less in agreement with the frequency of waves is the natural period of the ship. In other words, if the waterplane area is small, the heaving motion of the ship is reduced.
As pointed out hereinbefore, the draft line of the semi-submerged ship is on the struts 2. Accordingly, the waterplane area on the struts, that is, the sectional area of the draft line portion, is smaller than in an ordinary ship (as compared based on the same displacement volume), and the natural period is longer. Therefore, the natural period of the semi-submerged ship is hardly synchronous with the frequency of waves in an ordinary sea climate and hence, the pitching motion is reduced and a good stability is attained.
In view of the foregoing relation between the waterplane area and the natural period of motion, there has been proposed a structure in which the strut 2 is divided into front and rear parts, as shown in FIG. 3, to reduce the waterplane area. In this structure, however, the wavemaking resistance is remarkably increased by interference of the front and rear struts during navigation. Moreover, since the entire length of the strut 2 is shortened, it is difficult to ensure a required displacement. Accordingly, this structure is not preferred from the practical viewpoint.
Furthermore, since a semi-submerged ship having the above-mentioned structure has a peculiar shape quite different from the shape of an ordinary ship, designing is complicated and difficult. In order to utilize or manifest excellent technical characteristics of a semi-submerged ship sufficiently, the configurations and dimensions of the lower hulls or struts should be appropriately set. Furthermore, since the configurations and dimensions of the lower hulls or struts have significant influences on the speed characteristics and navigation properties, it is required to establish a highly developed designing technique constructed while taking the respective properties and characteristics collectively into account.